In U.S. Pat. No. 4,188,122 there is described a digital heterodyne interferometer which is useful in a analyzing the surface contour of a mirror, for example, utilizing a Twymann-Green interferometer. A test beam entering the interferometer is separated into two components which differ in frequency by a very small amount, e.g., a few KHz. The two components have different polarizations for purposes of identification. After passing through the interferometer, the components are processed to provide of the same polarization so that the components, after passing through the interferometer are permitted to interfere. Interference fringes can be observed either visually or electronically in a plane intersecting the output beam of the interferometer. Interference fringes move in a direction perpendicularly to their extensions at a rate equal to the difference frequency of the two input components to the interferometer. Thus the light intensity oscillates at any point in the detection plane at the difference frequency. The phase at any point in the detection plane is directly proportional to the optical path difference between the two arms of the interferometer, which in turn is due to imperfections or aberrations in the surface of the mirror being tested relative to a reference mirror surface.
The present invention utilizes this heterodyne interferometer concept to analyze aberrations in the wavefront of a beam being tested or analyzed by measuring phase variations from point to point in a detection plane at the output of the interferometer. The heterodyne wavefront analyzer of the present invention is a passive device requiring only a wavefront to be received.